The present invention relates generally to CT detector design and, more particularly, to a CT detector with non-rectangular detector cells.
In conventional multi-row CT detectors, a two dimensional array of detector cells extend in both the x and z directions. Moreover, in conventional detectors, each cell of the array is constructed to have a rectangular-shaped active area. This active area is generally perpendicular to a plane of x-ray source rotation and, in the context of energy integrating scintillators, converts x-rays to light. The light emitted by each scintillator is sensed by a respective photodiode and converted to an electrical signal. The amplitude of the electrical signal is generally representative of the energy (number of x-rays x energy level of x-rays) detected by the photodiode. The outputs of the photodiodes are then processed by a data acquisition system for image processing.
As described above, each of the detector cells of the 2D array has a generally rectangular or square face, and is contiguous in both the x and z directions. As such, there is no overlapping in either of the x or z directions. This lack of overlapping places an upper limit on the spatial frequency of the region-of-interest, i.e., anatomy of interest, which can be resolved artifact free. A number of approaches have been developed to overcome the upper sampling limitations of conventional 2D detector arrays.
In one proposed solution, miniaturization efforts have led to a reduction in the size of the individual detector cells or pixels. Because the output of each detector cell corresponds to a pixel in a reconstructed image; conventionally, detector cells are also referred to as pixels. Segmenting the detector active area into smaller cells increases the Nyquist frequency but with the added expense of data channels and system bandwidth. Moreover, system DQE is degraded due to reduced quantum efficiency and increased electronic noise which results in a degradation in image quality.
In another proposed technique, focal spot deflection by deflecting the x-ray focal spot in the x and/or z direction at 2× or 4× the normal sampling rate has been found to provide additional sets of views. The different sets of views are acquired from slightly different perspectives which results in unique samples that provide overlapping views of the region-of-interest without subpixellation. A drawback of this approach is that a data acquisition system channel capable of very high sampling rates is required. Moreover, such a technique requires an x-ray source and associated hardware dedicated to rapid beam deflection. Ultimately, it has been found that focal spot deflection yields images with increased noise and reduced dose efficiency.
Another proposed approach to increasing sampling density of a CT detector involves the staggering of pixels. Specifically, it is has been proposed that sampling density may be improved by offsetting, in the z direction, every other channel or column of detector cells in the x direction. In one proposed approached, the offset is equal to one-half of a detector width. This proposed CT detector design as well as a more conventional CT detector design are illustrated in FIGS. 1-2.
As shown in FIG. 1, a conventional CT detector 2 is defined by a 2D array of detector cells 3 that are rectangular in their active area shape. As shown and described above, the array extends in both the x and z directions. In the CT detector design illustrated in FIG. 2, every other channel 4 (column) of detector cells 3 is offset. This provides an intermediate sample location between rows 5 increasing the number cells, decreasing cell size, or increasing the data acquisition system sampling rate. However, such a staggered design is difficult to fabricate since all the rows are not aligned.
Therefore, it would be desirable to design a CT detector that provides increased sampling density that is practical to fabricate yet does not over-burden the data acquisition system or necessitate an impractical number of data acquisition channels.